An extrusion machine or extruder typically includes a screw element received within a cylindrical sleeve or barrel. The screw element includes one or more flights or threads arranged along the length of the element. The screw element is rotated within the sleeve while material such as e.g., one or more rubbers or plastics are fed into one end of the sleeve. Rotation of the screw element masticates and heats the material while pushing the material through the sleeve. A die plate positioned at the exit of the extruder can be used to impart a particular shape to the material as it passes through one or more openings in the die plate due to the substantial pressure created by rotation of the screw element.
By way of example, in modern processes for the manufacture of tires, the extruder may be used to create a layer of rubber for the tread portion of the tire. The layer of rubber is commonly created from a complex mix of materials fed into the extruder that can include various elastomers, resins, carbon black fillers, non-carbon black fillers, and/or other substances. The extruder functions to process the materials under heat and pressure for use as tread while the die plate provides the desired profile for the extruded tread portion.
One conventional construction for an extruder screw can include one or more flights uniformly spaced along a cylindrical core or screw root. Either a constant or variable pitch is maintained between the flights, and the core maintains a constant or variable diameter of a circular profile along its length. The cylindrical sleeve into which the extruder screw is received may be featureless or may include one or more flutes.
Unfortunately, such conventional extruder screw constructions do not provide the level of processing needed for modern rubber formulations at the rate of flow or throughput of material desired. Difficulties can be encountered in properly masticating, heating, and flowing some rubber formulations within the extruder in a manner that allows the material to be properly shaped with a typical extrusion die at the rate of throughput needed for economical manufacturing. Instead, for example, the overall rate of throughput must typically be slowed substantially in order to obtain the required amount of heat, homogenization, and mastication. A lower rate of throughput is undesirable as it slows the manufacturing process thereby increasing costs.
Accordingly, an extruder having improved performance capabilities would be useful. More particularly, an extruder screw capable of providing the desired level of material processing at an acceptable rate of throughput of the material would be beneficial. Such an extruder screw that can perform with modern rubber formulations would also be particularly useful.